In a large nonstick skillet coated with cooking spray, brown beef on both sides. Place onion and green pepper in a 3 qt slow cooker. Top with beef. Combine the broth, Worcestershire sauce, dill, thyme, pepper and pepper flakes; pour over beef. Cover and cook on HIGH for 3-4 hours or until meat reaches desired doneness and vegetables are crisp-tender. Remove beef and keep warm. Combine cornstarch and water until smooth; gradually stir into cooking juices. Cover and cook about 30 minutes longer or until slightly thickened. Return beef to the slow cooker; heat through.